In one presently known form of parking brake for use exclusively with disc brakes, a locking cylinder is hung between the tongs of the disc brake unit and is thereby actuated by the tongs during a normal service brake application. In another form for use both with disc-type or tread-type brakes, the locking cylinder is built integrally with the high pressure brake cylinder. In either case, the parking brake can then be set, following a brake application which brings the vehicle to a stop, simply by applying pressure to the locking port of the locking cylinder to cause the brake application to be locked up.
Normally, with the type of parking brakes above described, it is necessary to provide each of the brake units with a parking brake. Since on some types of vehicles such as those used for mass transit, it is not necessary to provide each brake unit with a parking brake, it would be desirable to provide parking brakes only on certain units necessary to provide the parking function and to make the parking brakes actuable independently of the service brake.